Platinum nanoparticles with continuously tunable mesoporous structures were prepared by a simple, one-step polymeric approach. By virtue of their large pore size, these structures have a high surface area that is accessible to reagents. In the synthetic method, variation of the solvent composition plays an essential role in the systematic control of pore size and particle shape. The mesoporous Pt catalyst exhibited superior electrocatalytic activity for the methanol oxidation reaction compared to commercially available Pt catalysts. This polymeric-micelle approach provides an additional design concept for the creation of next generation of metallic catalysts.